Anodizing process for the metal beryllium



United States Patent 3,276,974 ANODIZING PROCESS FOR THE METAL BERYLLIUMRichard C. Tyson H, San Jose, Calif., ,assignor, by mesne assignments,to the United ,States of America as represented by the Secretary of theNavy No Drawing. Filed Jan. 18, 1963, Ser. No. 252,538

1 Claim. (Cl; 20432) This invention relates to the metal coating art andhas particular reference to the productionof a protective coating uponberyllium during an anodizing treatment thereof.

Beryllium is a metal possessing a remarkable combination of propertieswhen compared to other metals. It exhibits an elastic modulus 40% higherthan steel, a strength-weight ratio 30% greater than titanium, a densitycomparable to magnesium, a 40% conductive capacitance for heat andelectricity as that of copper. The main difliculty with beryllium is itsquick susceptibility both to stress corrosion cracking and also togalvanic corrosion when subjected to a corrosive atmosphere such as asalt atmosphere.

Stress corrosion cracking is caused by the combined action of acorrosive media and of a residual or applied stress. The path of thecracking follows a trans-granular or inter-granular course at the grainboundaries, slip planes, planes of precipitated constituents or in anysuch area which differs from the grain and which becomes cathodic oranodic to the grain. Since these zones are in electrical contact withthe grain, a galvanic couple is established whereby the metal at theanode becomes soluble. As this electrochemical reaction takes place, anapplied or residual stress pulls the metal away from each side of theelectrolytic zone thereby creating a fissure. Since new material isbeing continually exposed, the crack proceeds preferentially throughoutthe area.

Galvanic corrosion is a localized attack which occurs when dissimilarmetals are in contact and exposed to corrosive medium. The dissimilarmetals function as short circuited electrodes. The corrosive medium canbe a moisture, salt spray or other such atmosphere which acts as anelectrolyte. A small galvanic current is induced and the metal havingthe more anodic potential is dissolved.

Under the prior art many attempts were made to coat beryllium with thinprotective layers of suitable metals which would either alleviate oreliminate entirely the above discussed corrosion problem peculiar to themetal beryllium. Such methods comprise among others those ofelectroplating, roll cladding, deposition from a carboxyl and dipcoating. All of the methods employed under the prior art failed toprovide the metal beryllium with a corrosion-resistant coating whichwill give adequate protection at elevated temperature against eithergalvanic or stress corrosion. This is because of the tendency ofberyllium to form hard, brittle inter-metallic compounds with the morecommon metals which are used for coating metal surfaces. It has beendetermined that galvanic action takes place at the metal-berylliumfaying surface.

In addition to this difiiculty, electroplated beryllium forms brittlediffusion phases at high temperature thereby resulting in the platedmetal being separated from the beryllium base. This produces anirregular coating and the resultant voids are diflicult or impossible tobridge during subsequent plating.

Therefore, one of the objects of this invention is to provide animproveed anodizing process for protecting a beryllium surface.

Another object of the invention is to provide a novel process for theproduction of oxide coatings upon beryl- 3,276,974 Patented Dot. 4, 1966lium which are substantially poreless and which withstand all types ofcorrosion.

A still further object of this invention is to provide a process forincreasing the corrosion abrasiveness of beryllium.

Other objects and advantages of this invention, it is believed, will bereadily apparent from the following detailed description thereof.

Briefly, this invention includes the discovery that the surfaces ofberyllium may be provided with an oxide film brought about by carefullycontrolling the composition of the bath, the bath temperature, the timeand the current density during an anodizing electrolytic deposition.Unlike aluminum, beryllium is a sintered metal which makes it difficultto clean prior to being coated. Beryllium has the additional problem ofcurrent b-urn due .to its high throwing power while it is in theelectrolyte. The present invention is directed to a process forovercoming these difliculties.

The invention will be illustrated by the following non limitativeexample.

The surface of a beryllium workpiece was first degreased withconventional degreasing agents such as gasoline, or mineral spirits orthe like. Beryllium should not be placed in pickling solutions as thesesolutions will produce preferential dissolution of various inclusions onthe surface of beryllium due to its sintered nature thereby leavingvoids in the surface. These resultant voids are difi'icult to bridgeduring the subsequent plating operation.

The workpiece was baked for one hour in a furnace wherein thetemperature is maintained between ZOO-220 F. in order to eliminateblistering of the film. It was then Water rinsed and cooled.

It is then given a chemico-electrical treatment in a bath having thefollowing composition and under the following conditions:

Electrolyte 8 oz./gal. of sodium carbonate, 2 oz./gal. of sodiumhydroxide in water. The solution to be heated to 130- 145 F.

Procedure The beryllium workpiece was immersed in the bath with theresult that the solution of the metal by the alkaline electrolytecommences almost immediately. Direct current of 30-45 .amperes sq. footwas switched on, the beryllium being the cathode. An iron or steel anodewas used and procedure in the bath follows electro-plating practice tosome extent. The treatment was continued under these conditions forabout 1.2 to 20 minutes. The current was then turned off and theberyllium was permitted to soak from 5 to 20 minutes in the solution.The temperature of the solution was maintained at 130-145 F. The current'was then turned on again and the metal cathode was subjected to thesame treatment with the current density being 30 to 45 .amperes per sq.ft. The article was then removed from the bath with the current still onand immediately rinsed in clean water. The surface of the metal nowpossessed a very bright appearance.

For the following part of the treatment, the metal was subjected to ananodic process in an electrolyte, the composition of which and theconditions of operation being as follows:

Chemical polish After any areas on the workpiece which are not to beanodized were masked, it was dipped for 20 to 50 minutes in a watersolution containing by weight of 85% H PO 7% by weight of CrO and 5% byweight of commercial H The temperature of this solution 3 was maintainedbetween 70 F. and 120 F. The-workpiece was again Water rinsed. It wasnow ready for the anodizing procedure.

- Electrolyte Aqueous solution containing 6 /2 to 8 oz./ gal. of CrOmaintained at a temperature of 60 to 75 F.

Procedure The beryllium workpiece was made the anode. The cathode can bemade of either iron, steel, graphite, a lead tin alloy or aluminum andbecause of the high throwing power characteristic of beryllium, thecathode is shaped so as to correspond to the design of the workpiece andplaced approximately 2 /2" to 3" away from the workpiece in theelectrolyte in order to avoid current burn. Holes are drilled in tthecathode to permit circulation of the electrolyte during'processing. Allof these cathodic preparations are necessary because of the extremelyhigh current density required to anodize beryllium. A current density of130 to 150 amperes/square foot was required, for a period of 30 minutesto obtain a coating of beryllium oxide .001 to .002 inch thick on thebase metal. This is very significant when compared 5 A sealing operationwas'performed' on the workpiece by placing it in an aqueous solution ofsodium chromate (.12 to .24 ounce/gal. of Na Cr O )-fOr 10 to 15minutes. The temperature of the solution was maintained between 160 and185 F.

The film produced according to this method-has a dielectric constant ofapproximately 6.3 which is better than either magnesium oxide oraluminum oxide. Its Brinell hardness is 112-114 and it was able towithstand a salt spray test in excess of 200 hours without showing anyevidence of corrosion or surface deformation.

While the foregoing particular example deals only with pure beryllium,the scope of the invention is not limited thereto and beryllium alloysare included in the materials which are susceptible to beneficialtreatment by the process according to .the invention. The abovedescription and example are intended to be illustrative only. Any

modification of or variation therefrom which conforms to the spirit ofthe invention is intended to be included within the scope of the claim-What is claimed is:

A process for treating the surface of an article ofberyllium, includingthe steps in sequence of:

(a) degreasing the surface with gasoline;

(b) baking the surface for one hour at a temperatur 4 sodium carbonateand sodium hydroxide in the proportions of about 8 ounces/gal. of thesodium carbonate and 2 ounces/gal. of the sodium hydroxide at atemperature of 130 to 145 F.;

(e) applying direct. current having a density of the order of 30 .to 45amperes per square foot of the surface through the said sodium carbonatesodium hydroxide solution for 12 to 20 minutes, the beryllium surfacebeing the cathode in the. circuit;

(f) soaking the surface in the said solution 'without the application ofthe direct current for 5 to 20 minutes;

(g) reapplying the direct current through the said solution with acurrent density of 30 to 45 amperes per square foot of the surface for12 to 20 minutes;

(h) water rinsing the surface;

(i) polishing the surface in an aqueous solution consisting essentiallyof by weight H PO 7% by weight'of CrO and 5% by weight of H for 20 to 50minutes at a temperature of 70 to F.;

(j) water rinsing the surface;

(k) anodizing the surface in an aqueous soution of chromic oxidecontaining 6% to 8 ounces per gallon of CrO at a temperature of 60 to 75F.W1l.h a current density of to amperes per square foot of the surfacefor a period of 30 minutes; the

r cathode in the anodizing treatment having openings therein and beingdisposed 2% to 3 inches away from the berylliumanode surface;

'- (1) water rinsing the surface;

(m) and sealing the anodized surface in an aqueous solution containing.12 to .24 ounce per gallon of Na Cr O at a temperature of to F. for 10to 15 minutes.

References Cited by the Examiner OTHER REFERENCES Graham, ElectroplatingEngineeering Handbook 1955, Reinhold Pub. Co., ppp143-6, 161, 376-80.

Levin, Trans. Faraday Soc. 54; 935-40 (June 1958). Tech. Proc. Am.Electroplaters Soc. 48; 106-8, 109-11 (1961).

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, Examiner.

L. G. WISE, W. VANSISE, Assistant Examiners.

